A blood vessel is expanded with a balloon catheter to indwell a stent therein for treatment of diseases relating to the disorder of blood vessels such as myocardial infarction or cerebral infarction. While stents made of metal material are commonly used as such stents, these metal stents are retained in the body permanently. Due to this, a stent made of metal material cannot be applied to a person not in maturity whose physical structure of the body is changed over time, and there is a risk of recurrence of stenosis due to long-term mechanical stimulation.
Stents made of polymer materials do not have such defect as compared with the stents made of metal materials. Stents made of biodegradable polymers have an advantage that solves the problem of stress caused by the stent that is retained in the body permanently, so they are frequently utilized in recent years.
Techniques regarding stents made of polymer material are disclosed in the following documents. Patent Document 1 discloses a ladder-shaped expandable stent formed by connecting a plurality of ladder elements serially, in which each ladder element enables two longer ribs to slide, the distance between the end side cross rails in neighboring ladder elements is made variable so that the stent can be expanded from a reduced diameter to an expanded diameter and slide back to a reduced diameter is inhibited. Patent Document 2 discloses a stent used in lumens formed of a cylindrical sheet that has a series of protrusions and holes locking each other and provides overlapping end sides so as to create ratchet action when the stent is brought to an expanded state for supporting a portion of an inner wall of a lumen.
Patent Document 3 discloses a stent made of polymer material formed by connecting a plurality of T-shaped units, each of which consists of a head portion and an elongated body portion extending from the head portion, such that one side or both sides of each body portion has at least one protrusion and the head portion has an opening for allowing the body portion to pass through and latching its protrusion.
Patent Document 4 discloses a stent made of polymer material formed by connecting a plurality of T-shaped units, each of which consists of a head portion and an elongated body portion extending from the head portion, such that one side of each body portion has ratchet protrusions and the head portion has a slit for allowing the body portion to pass through and latching its ratchet protrusions, wherein movement of the body portion towards reduction of the diameter of the stent is inhibited and movement of the body portion towards expansion of the diameter of the stent is allowed, either by providing a width changeable portion on the other side of the body portion or by making the connecting portion around the slit in the head portion elastically deformable to change the tilt angle of the slit.
The ladder-shaped expandable stent in Patent Document 1 has a complex shape and structure and has a possibility that the diameter of the stent cannot be varied easily because of large resistance applied on the long ribs. Further, tab stops are provided on the longer ribs to prevent the longer ribs from sliding in the direction that the diameter of the stent is to be reduced and the end side cross rails are engaged to the tab stops, whereas the longer ribs and the end side cross rails are arranged to be at a right angle. For this sake, action of the tab stops induced by movement of the longer ribs may cause the end side cross rails to be distorted and cause the stent to be deformed. The stent according to Patent Document 2 is formed to be a cylindrical sheet, so that the rigidity of the stent is high. Further, ends of the cylindrical sheet protrude outwards, so that it is difficult to make its sectional shape to be circular in a lumen and there is a possibility of being unable to stick fast to the inner wall of the lumen.
Patent Document 3 is proposed by the present inventor to solve problems such as in Patent documents 1 or 2. According to this, strong pressure is applied to the stent made of polymer material from its inside when it is expanded by a balloon and the head portion is placed on the body portion at the site where the body portion pass through the slit. For this sake, deformation of the stent occurs with friction at this site and pressure from inside to cause the linear cut line continuing from the slit to be deformed with expansion so that there is a possibility in which ratchet protrusions are not locked by the slit and movement towards the direction of reduction of the diameter of the stent cannot be withstood, that is, function of the stent is impaired.
Patent Document 4 is proposed by the present inventor, making further improvement. According to this, movement of the body portion towards reduction of the diameter of the stent is inhibited and movement of the body portion towards expansion of the diameter of the stent is allowed, by providing a width changeable portion on the side of the body portion where ratchet protrusions are not formed, by making the tilt angle of the slit in the head portion elastically variable or by making the connecting portion around the slit in the head portion elastically deformable. On the other hand, this is similar to Patent Documents 1 to 3 in that the stent is formed by winding up a sheet material of stent into a cylindrical stent. Degree of freedom in the direction normal to the sheet face is high in stents formed by winding up a sheet material of stent into a cylindrical stent, so that there is a possibility of being unable to stably lock the stent when force in this direction is applied in a case where the stent is used for a long time.
Further, as a common case, a stent is to be indwelled in a main vessel and a branch vessel, when the stent is applied to a site of disease at a branched portion of a blood vessel. T-stent operation, as a method for indwelling a stent in a branched portion of a blood vessel, is a simple technique consisting of indwelling a stent in a branch vessel for first and then indwelling another stent in a main vessel, after which both stents are expanded simultaneously. While this operation is adapted to the case where the angle of the branch vessel is no less than 70 degrees, typically near 90 degrees, there is a disadvantage such that an area is created that cannot be covered with the stent at the site entering the branch vessel when the angle of the branch vessel is small.
Culotte stent operation will be explained referring to FIG. 12 (a) to FIG. 12(c).
FIG. 12 (a) shows a state where a stent S1 is indwelled at a branched portion in a main vessel of a blood vessel so as to extend covering the branched portion. Then, as shown in FIG. 12 (b), another stent S2 is introduced to the branch vessel, widening the struts at the position of the side of the stent S1 facing the branch vessel with the balloon catheter mounting the another stent S2. After this, by expanding both stents simultaneously, a state where stents are indwelled in the main vessel and the branch vessel respectively is attained, as shown in FIG. 12 (c). This technique, enabling the portion entering into the branch to be fully covered with the stents, is adapted also to the case where the angle of the branch vessel is small.
When a stent is to be indwelled in a branch vessel of a blood vessel in such a manner as introducing another stent to the branch vessel, pushing aside the struts on the side of the stent, and the stent is one made of metal material, the configuration of the stent is preserved with plastic deformation and the struts are bent to have a configuration enabling further expansion exceeding the diameter of the blood vessel. In contrast to this, if the stent is one made of polymer material, the struts become to be a straight elongated state when the stent is expanded to the diameter of the blood vessel, so that it is not possible to widen the struts on the side of the stent. This situation is similar in case of a stent made of polymer material as disclosed in Patent Documents 1 to 4. Thus, these are not adapted to indwelling a stent at a branched portion of a blood vessel.